1. Field of the Invention
The present invention relates to a signal processing method and apparatus, and in particular to a signal processing method and apparatus when processing such as a noise suppression is performed to a signal in a frequency domain and then the signal is returned to a time domain to be processed.
2. Description of the Related Art
Prior art examples [1] and [2] of a signal processing technology as mentioned above will now be described referring to FIGS. 14-17.
Prior Art Example [1]: FIGS. 14 and 15
A noise suppressing apparatus 2 shown in FIG. 14 is composed of a frame division/windowing portion 10 which divides an input signal In(t) that is a voice signal into units of a predetermined length and performs a predetermined window function, a frequency spectrum converter 20 which converts a windowed frame signal W(t) outputted from the frame division/windowing portion 10 into a frequency spectrum X(f) composed of an amplitude component |X(f)| and a phase component argX(f), a noise suppressing portion 130 which performs a noise suppression to the amplitude component |X(f)| of the frequency spectrum X(f), a time-domain converter 40 which converts the amplitude component |Xs(f)| after the noise suppression and the phase component argx(f) of the frequency spectrum X(f) into the time domain, and a frame synthesizing portion 60 which synthesizes a time-domain frame signal Y(t) outputted from the time-domain converter 40.
FIG. 15 shows an operation waveform diagram of the noise suppressing apparatus 2. Firstly, the frame division/windowing portion 10 sequentially divides the input signal In(t) into a last frame signal FRb(t) and a present frame signal FRp(t) (hereinafter, occasionally represented by a reference character FR) of a predetermined frame length L. The frame signals FRb(t) and FRp(t) are deviated by frame shift length ΔL and cut out from the input signal In(t) so that the parts of the signals may be overlapped with each other, in order to more accurately perform processing for noise suppression (namely, in order to more minutely analyze the frequency spectrum), which will be described later.
Furthermore, the frame division/windowing portion 10 sequentially performs a predetermined window function w(t) to the frame signals FRb(t) and FRp(t) according to the following Eq.(1) to output the windowed frame signal W(t) (at step T1).·W(t)=FR(t)*w(t) (t=0−L)  Eq.(1)
This window function w(t) is set, as shown in FIG. 15 for example, so that the amplitudes of both ends of the frame signals FR(t) may become equally “0” and the sum of mutual contribution degrees at the overlapping portion of the frame signals FR(t) may become “1”.
The operation of the frequency spectrum converter 20, the noise suppressing portion 130, and the time-domain converter 40 will now be described by taking the windowed frame signal Wb(t) obtained corresponding to the last frame signal FRb(t) for example. This can be similarly applied to the windowed frame signal Wp(t) corresponding to the present frame signal FRp(t).
The frequency spectrum converter 20 converts the windowed frame signal Wb(t) into the frequency spectrum X(f) by using an orthogonal transform method such as MDCT (Modified Discrete Cosine Transform) and FFT (Fast Fourier Transform), provides the amplitude component |X(f)| to the noise suppressing portion 130, and provides the phase component argX(f) to the time-domain converter 40.
The noise suppressing portion 130 suppresses the noise component included in the amplitude component |X(f)|, and provides the amplitude component |Xs(f)| after the noise suppression to the time-domain converter 40 (at step T2).
The time-domain converter 40 having received the phase component argx(f) of the frequency spectrum X(f) and a noise suppressed amplitude component |Xs(f)| provides a time-domain frame signal Yb(t) obtained by the conversion into the time domain (reverse orthogonal transform) to the frame synthesizing portion 60 (at step T3).
The frame synthesizing portion 60 having received the time-domain frame signal Yb(t) and a time-domain frame signal Yp(t) corresponding to the present frame signal FRp(t) similarly obtained synthesizes or adds the time-domain frame signals Yb(t) and Yp(t) as shown by the following Eq.(2) to obtain an output signal Out(t) (at step T4).
                                                                                                   ·                                      Out                    ⁡                                          (                      t                      )                                                                      =                                ⁢                                                      Y                    ⁡                                          (                                              t                        -                                                  Δ                          ⁢                                                                                                          ⁢                          L                                                                    )                                                        +                                      Y                    ⁡                                          (                      t                      )                                                                                                                                              =                                ⁢                                                      Yb                    ⁡                                          (                      t                      )                                                        +                                      Yp                    ⁡                                          (                      t                      )                                                                                                                              Eq        .                                  ⁢                  (          2          )                    
Thus, it becomes possible to obtain the output signal Out(t) in which the noise component is suppressed, from the input signal In(t).
However, the amplitude at each end of the frame of the time-domain frame signal Yb(t) or Yp(t) becomes larger or smaller than “0” as shown in FIG. 15 due to the noise suppression at the above-mentioned step T2, so that the amplitudes of the frame end are mutually deviated in some cases. In these cases, there is a problem in this prior art example [1] that the output signal Out(t) becomes discontinuous at boundaries B1 and B2 of the time-domain frame signals Yb(t) and Yp(t), so that abnormal noise is generated.
In order to address this problem, the following prior art example [2] has already been proposed.
Prior Art Example [2]: FIGS. 16 and 17
The noise suppressing apparatus 2 shown in FIG. 16 is provided with a post-windowing portion 140 which is connected between the time-domain converter 40 and the frame synthesizing portion 60, and which outputs a post-windowed frame signal Wa(t) in which a post-window function is performed to the time-domain frame signal Y(t), in addition to the arrangement shown in the above-mentioned prior art example [1].
In operation, as shown in FIG. 17, the post-windowing portion 140 sequentially performs a predetermined post-window function wa(t) to the time-domain frame signals Yb(t) and Yp(t) obtained in the same way as the above-mentioned prior art example [1] according to the following Eqs.(3) and (4) to output the post-windowed frame signals Wab(t) and Wap(t) (at step T5).·Wab(t)=Yb(t)*wa(t)  Eq.(3)·Wap(t)=Yp(t)*wa(t)  Eq.(4)
The post-window function wa(t) is set so that the amplitudes of both ends of the time-domain frame signals Yb(t) and Yp(t) may become “0” again as shown in FIG. 17 (i.e. so that the amplitudes may become continuous at the boundaries B1 and B2 of the time-domain frame signals Yb(t) and Yp(t)).
The frame synthesizing portion 60 synthesizes or adds the post-windowed frame signals Wab(t) and Wap(t) as shown in the following Eq.(5) to obtain the output signal Out(t) (at step T6).
                                                                                             ·                                      Out                    ⁡                                          (                      t                      )                                                                      =                                ⁢                                                      Wa                    ⁡                                          (                                              t                        -                                                  Δ                          ⁢                                                                                                          ⁢                          L                                                                    )                                                        +                                      Wa                    ⁡                                          (                      t                      )                                                                                                                                              =                                ⁢                                                      Wab                    ⁡                                          (                      t                      )                                                        +                                      Wap                    ⁡                                          (                      t                      )                                                                                                                                Eq          .                                          ⁢                      (            5            )                              
Thus, it becomes possible to obtain the output signal Out(t) in which the time-domain frame signals Yb(t) and Yp(t) are continuously connected at the boundaries B1 and B2 (see e.g. patent document 1).
It is to be noted that as a reference example, an echo suppressing apparatus can be mentioned which connects the frame signals obtained by converting the frequency spectrum to which an echo suppression is performed into a time domain by using the post-window function in the same way as the above-mentioned prior art example [2] (see e.g. patent document 2).    [Patent document 1] Japanese patent No. 3626492    [Patent document 2] Japanese patent application laid-open No. 2000-252891
In the above-mentioned prior art example [2], it is possible to continuously connect the frame signals after the correction by sequentially correcting the frame signals by using the post-window function. However, since the amplitude component of the frame signal is multiplied by the post-window function, in other words, since the amplitude component |Xs(f)| corresponding to all of the frequency components included in the frame signal are corrected, as shown in FIG. 18, there is a problem that the frequency spectrum amplitude component |Xa(f)| (shown by a solid line) of the frame signal Wa(t) after having taken the post-window function processing becomes blunt in the whole frequency bandwidth compared with the frequency spectrum amplitude component |Xs(f)| (shown by dotted line) of the frame signal Y(t) before taking the post-window function processing, so that a distortion is generated in the entire frame signal.
Generally, it is considered that a hearing sensitivity in a high frequency bandwidth whose frequency “f” is 20 Hz-20 kHz is high. Therefore, a distortion in the frame signal generated in the high frequency bandwidth leads to a deterioration of a sound quality.